Electronic circuits, as well as e.g. micro-mechanical and/or optical structures and opto-electronic devices, can be integrated in a chip obtained from a semiconductor material wafer by means of a process typically executed in a sequence of stages. At the end of such a process the wafer includes a plurality of chips, or dice, which are then separated by sawing; typically, the chips are then embedded in packages suitable for their use.
In the case in which the chips are found to be defective or subject to failures during operation and are returned to the manufacturer for analysis of the failure, the possibility of recovering the original position of the chips in the wafer before sawing is of strategic importance for quality management of the manufacturing process. In fact, functional (e.g. electrical or optical) parameters, performance and reliability of each chip can vary with the position in the wafer, for example because the crystallographic quality of the wafer material can vary with the position in the wafer. Accordingly, analysis on failure occurrences is very useful for allowing the manufacturers to devise strategies for improving the chip quality.
For this purpose, an indication of such a position is provided on each chip, i.e., a so-called die (or chip) indexing is exploited for recovering the die position in the wafer.
In the art, die indexing methods are known in which a visible sign is provided on each chip during the fabrication process, e.g. on an upper passivation layer in the stack of material layers of the wafer, in such a way that it is possible to read a die index by non-invasive inspection also when the chip has failed. For example, the index can be a respective number of a number sequence, assigned to each chip of a wafer, provided on a boundary portion of the chip.
A process of fabricating chips from a wafer is executed in different stages in which a plurality of masks is usually exploited for defining desired patterns in layers of the wafer. Roughly speaking, a mask is a flat plate containing photographic images (i.e., transparent and opaque areas) of structures useful to implement, for example, the integrated circuits of the chips on wafer material layers.
At prescribed stages, the wafer is coated with a layer of a photosensitive material, typically referred to as photosensitive resist or photoresist, and a mask, properly aligned with the wafer, is illuminated by an optical system with light in a range of wavelengths suitable for exposing the photoresist. Thus, the images on the mask are projected and transferred onto the photoresist layer coating the wafer. The exposed photoresist is then developed to produce a pattern on the wafer which identifies areas of material layers to be etched (one such sequence of the fabrication process is termed photolithography).
The fabrication process can be performed by means of traditional projection equipment, comprising the optical system, in which the wafer is positioned and aligned with the masks necessary for the fabrication process. As long as the wafer dimension is relatively small, such masks include a number of images equal to the number of chips to be obtained from the wafer and the whole wafer is exposed through that mask in one go.
Typically, methods of indexing exploiting the projection equipment require that one of the plurality of masks includes the images of the indexes of all the chips formed on the wafer.
The increase in wafer size has led to the introduction of projection equipment employing so-called wafer steppers, in which the images of the mask useful to manufacture the chips are projected onto portions of the wafer “step by step”, instead of in a single go. A wafer stepper includes the optical system and an appropriate alignment system for aligning the optical system, the mask and the wafer in a “step by step” fashion. In the case in which the wafer stepper is used, the projected image of the mask typically covers an area which is a portion of the total wafer area. After each exposure, the wafer in the wafer stepper is moved under the optical system by exactly the size of the projection of the mask on a plane of the wafer. The wafer steppers make it possible to obtain a plurality of chips from wafers larger than (typically, of eight inches) the wafers traditionally used with a conventional projection equipment (of maximum six inches). Regretfully, this makes the traditional masks exploited with a projection equipment not compatible to the substantially larger wafers manufactured by means of a wafer stepper and, accordingly, using the wafer steppers it is not possible to project the images of the indexes of all the chips formed on the wafer in one go.
Alternative methods of die indexing exploit non-volatile storage elements, integrated in the chips, such as EPROM devices, for storing information relating to die coordinates indicative of the die positions in the wafer, written therein during testing, for example. However, such methods increase costs of the fabrication process, by adding service storage elements in the chips, and thus semiconductor area (non-volatile storage elements occupy portions of an intended active area of the chip in which, for example, the integrated circuits have to be implemented) and, possibly, dedicated process stages. In addition, such methods make the chips even more prone to defects, and in case the failed chips undergo functional failures, the stored information on the die coordinates may be not recoverable.